Yeast, Saccharomyces cerevisiae, which is a fermentation microorganism for a production process of cellulose ethanol, is not able to utilize xylose included in vegetable biomass in a large amount. Therefore, researches for imparting xylose utilization ability to Saccharomyces cerevisiae are in progress. To this end, introduction of 2 types of pathways to the yeast is investigated. One is a pathway (XR-XDH pathway) using a xylose reductase (XR) and a xylitol dehydrogenase (XDH). However, there is a drawback in the pathway that intermediate metabolites accumulate and the ethanol yield decreases. Meanwhile, in the case of a pathway (XI pathway) using a xylose isomerase (XI), there is no such a drawback, but another drawback arises that the consumption rate of xylose is slow in comparison to the XR-XDH pathway. Therefore, various investigations are under way for a high performance XI.
Improvement of a XI originated from Piromyces sp. E2, which was reported as the first XI being able to function in yeast, has been carried out (Patent Literature 1, Non Patent Literature 1). Further, improvement of a XI originated from Ruminococcus flavefaciens has been also carried out (Patent Literature 2). Further, improvement of a XI originated from Lactococcus lactis has been also carried out (Patent Literature 3). Furthermore, a XI originated from an enteric protist of Reticulitermes speratus having higher xylose consuming ability of yeast compared to a heretofore known XI has been reported (Patent Literature 4).